This invention relates to the measurement of brain electrical activity.
Such measurement is based on the localized discrete sampling, both in space and time, of a biological variable. In multichannel evoked potentials (EPs) the underlying biological event is both space and time-variant. To analyze the scalp-recorded activity generated by such events, spatiotemporal relationships must be explicit. This is readily apparent by inspecting the dynamic evolution of topographic maps generated by even simple sensory stimulations. Although topographic maps make biological events more comprehensible, they do not simplify the quantitative evaluation of the phenomenon; on the contrary, new and more complex features are made evident. Expressions such as slowing, lateralized, persistent, focal, and asymmetrical are often used to describe these complex phenomena. Such subjective terminology may be descriptive of the findings but is not easily amenable to a quantitative evaluation. There is a a need to quantify these subjective judgments. Doing so will enhance the diagnostic power of event-related potentials.
It is known in the art to use topographic mapping to aid in the clinical evaluation of evoked potential (EP) data. In general, single topographic images of evoked potential (EP) data are formed every 4 msec and then all 128 images are displayed in rapid, recurring sequence. Thus, the spatial distribution or positive (red) and negative (blue) activities can be viewed by this "cartooning" process over the entire 512 msec EP epoch. Normal subjects characteristically demonstrate symmetrical negative or positive foci that appear and disappear as a function of time, often moving in an anterior or posterior direction. Concurrent positive and negative foci may coexist. In the face of pathology, this pattern becomes distorted. Regions of abnormality may show either diminished or greatly augmented activities or both at different times. Topographic movies in pathology often demonstrate asymmetrical foci. Positive or negative potential "hills" may sweep along the medial-lateral axis rather than along the more usual A-P axis. Clinical experience has shown such movies to be of considerable value in the identification of pathology. Subtle shifts of the spatial distribution of a potential hill or in the trajectory of its sweep across the head may serve to define the location of a lesion.
It is known to compute the trajectory of an evoked potential of normal subjects to visual stimuli by plotting the center of gravities of the sequence of topographic maps representing the evoked potential. Sandini et al., "Topography of Brain Electrical Activity: A Bioengineering Approach," Med Prog. through Technology 10: 5-19 (1983).
Brain electrical activity mapping (BEAM) is a known diagnostic tool for detecting brain abnormalities. BEAM is described in U.S. Pat. No. 4,421,122; Duffy et al., "Brain Electrical Activity Mapping (BEAM): A New Method for Extending the Clinical Utility of EEG and Evoked Potential Data," Ann. Neurol., 5: 309-321 (1979); Duffy, Bartels, et al., Significance Probability Mapping: An Aid to the Topographic Analysis of Brain Electrical Activity," Electroenceph. Clin. Neurophysiol., 512: 455-462 (1981); Duffy, Topographic Mapping of Brain Electrical Activity, Butterworths (1986) (all incorporated by reference).